octeon: fix sysupgrade

Sysupgrade was failing due to incorrect replacement of backticks:
/sbin/sysupgrade: /lib/upgrade/platform.sh: line 101: syntax error: missing '))'

Fixes: 0bbfc3dff7 ("octeon: replace backticks by $(...)")
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>

target/linux/octeon/base-files/lib/upgrade/platform.sh

@@ -85,8 +85,8 @@ platform_check_image() {
 	er | \
 	erlite)
 		local tar_file="$1"
-		local kernel_length=$((tar xf $tar_file sysupgrade-$board/kernel -O | wc -c) 2> /dev/null)
-		local rootfs_length=$((tar xf $tar_file sysupgrade-$board/root -O | wc -c) 2> /dev/null)
+		local kernel_length=$(tar xf $tar_file sysupgrade-$board/kernel -O | wc -c 2> /dev/null)
+		local rootfs_length=$(tar xf $tar_file sysupgrade-$board/root -O | wc -c 2> /dev/null)
 		[ "$kernel_length" = 0 -o "$rootfs_length" = 0 ] && {
 			echo "The upgrade image is corrupt."
 			return 1